第 11 章 直接内存

时间:2020-10-13 07:43:00 来源:互联网 作者: 神秘的大神 字体:

第 11 章 直接内存

1、直接内存概述

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直接内存

  1. 不是虚拟机运行时数据区的一部分,也不是《Java虚拟机规范》中定义的内存区域。
  2. 直接内存是在Java堆外的、直接向系统申请的内存区间。
  3. 来源于NIO,通过存在堆中的DirectByteBuffer操作Native内存
  4. 通常,访问直接内存的速度会优于Java堆。即读写性能高。
  5. 因此出于性能考虑,读写频繁的场合可能会考虑使用直接内存。
  6. Java的NIO库允许Java程序使用直接内存,用于数据缓冲区

代码示例

  • 代码

public class BufferTest {
    private static final int BUFFER = 1024 * 1024 * 1024;

    public static void main(String[] args){

        ByteBuffer byteBuffer = ByteBuffer.allocateDirect(BUFFER);
        System.out.println("直接内存分配完毕,请求指示!");

        Scanner scanner = new Scanner(System.in);
        scanner.next();

        System.out.println("直接内存开始释放!");
        byteBuffer = null;
        System.gc();
        scanner.next();
    }
}
  • 呐,直接占用了 1G 的本地内存

  • 释放后,Java程序的内存占用明显减少

2、BIO 与 NIO

非直接缓存区(BIO)

原来采用BIO的架构,在读写本地文件时,我们需要从用户态切换成内核态

直接缓冲区(NIO)

NIO 直接操作物理磁盘,省去了中间商赚差价

代码测试

  • 测试代码:分别使用 BIO 和 NIO 复制大文件,看看用时差别

public class BufferTest1 {

    private static final String TO = "F:\\test\\异界BD中字.mp4";
    private static final int _100Mb = 1024 * 1024 * 100;

    public static void main(String[] args) {
        long sum = 0;
        String src = "F:\\test\\异界BD中字.mp4";
        for (int i = 0; i < 3; i++) {
            String dest = "F:\\test\\异界BD中字_" + i + ".mp4";

            sum += directBuffer(src, dest);
        }

        System.out.println("总花费的时间为:" + sum);
    }

    private static long directBuffer(String src, String dest) {
        long start = System.currentTimeMillis();

        FileChannel inChannel = null;
        FileChannel outChannel = null;
        try {
            inChannel = new FileInputStream(src).getChannel();
            outChannel = new FileOutputStream(dest).getChannel();

            ByteBuffer byteBuffer = ByteBuffer.allocateDirect(_100Mb);
            while (inChannel.read(byteBuffer) != -1) {
                byteBuffer.flip();
                outChannel.write(byteBuffer);
                byteBuffer.clear();
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (inChannel != null) {
                try {
                    inChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
            if (outChannel != null) {
                try {
                    outChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
        }

        long end = System.currentTimeMillis();
        return end - start;

    }

    private static long io(String src, String dest) {
        long start = System.currentTimeMillis();

        FileInputStream fis = null;
        FileOutputStream fos = null;
        try {
            fis = new FileInputStream(src);
            fos = new FileOutputStream(dest);
            byte[] buffer = new byte[_100Mb];
            while (true) {
                int len = fis.read(buffer);
                if (len == -1) {
                    break;
                }
                fos.write(buffer, 0, len);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
            if (fos != null) {
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }

            }
        }

        long end = System.currentTimeMillis();

        return end - start;
    }
}

深入 ByteBuffer 源码

  • ByteBuffer.allocateDirect() 方法
public static ByteBuffer allocateDirect(int capacity) {
    return new DirectByteBuffer(capacity);
}
  • DirectByteBuffer 类的构造器用到了 Unsafe 类分配本地内存
DirectByteBuffer(int cap) {

    super(-1, 0, cap, cap);
    boolean pa = VM.isDirectMemoryPageAligned();
    int ps = Bits.pageSize();
    long size = Math.max(1L, (long)cap + (pa ? ps : 0));
    Bits.reserveMemory(size, cap);

    long base = 0;
    try {
        base = unsafe.allocateMemory(size);
    } catch (OutOfMemoryError x) {
        Bits.unreserveMemory(size, cap);
        throw x;
    }
    unsafe.setMemory(base, size, (byte) 0);
    if (pa && (base % ps != 0)) {

        address = base + ps - (base & (ps - 1));
    } else {
        address = base;
    }
    cleaner = Cleaner.create(this, new Deallocator(base, size, cap));
    att = null;

}

3、直接内存与 OOM

  1. 直接内存也可能导致OutofMemoryError异常
  2. 由于直接内存在Java堆外,因此它的大小不会直接受限于-Xmx指定的最大堆大小,但是系统内存是有限的,Java堆和直接内存的总和依然受限于操作系统能给出的最大内存。
  3. 直接内存的缺点为:
  • 分配回收成本较高
  • 不受JVM内存回收管理
  1. 直接内存大小可以通过MaxDirectMemorySize设置
  2. 如果不指定,默认与堆的最大值-Xmx参数值一致

代码示例 1

  • 代码

public class BufferTest2 {
    private static final int BUFFER = 1024 * 1024 * 20;

    public static void main(String[] args) {
        ArrayList<ByteBuffer> list = new ArrayList<>();

        int count = 0;
        try {
            while(true){
                ByteBuffer byteBuffer = ByteBuffer.allocateDirect(BUFFER);
                list.add(byteBuffer);
                count++;
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        } finally {
            System.out.println(count);
        }

    }
}
  • 本地内存持续增长,直至程序抛出异常:java.lang.OutOfMemoryError: Direct buffer memory

  • 异常信息
"C:\Program Files\Java\jdk1.8.0_144\bin\java" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\lib\idea_rt.jar=1296:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;C:\Users\Heygo\Desktop\JVMDemo\out\production\chapter11" com.atguigu.java.BufferTest2
180
Exception in thread "main" java.lang.OutOfMemoryError: Direct buffer memory
	at java.nio.Bits.reserveMemory(Bits.java:694)
	at java.nio.DirectByteBuffer.<init>(DirectByteBuffer.java:123)
	at java.nio.ByteBuffer.allocateDirect(ByteBuffer.java:311)
	at com.atguigu.java.BufferTest2.main(BufferTest2.java:21)

Process finished with exit code 1
</init>

代码示例 2

  • 代码:直接通过 Unsafe 类申请本地内存

public class MaxDirectMemorySizeTest {
    private static final long _1MB = 1024 * 1024;

    public static void main(String[] args) throws IllegalAccessException {
        Field unsafeField = Unsafe.class.getDeclaredFields()[0];
        unsafeField.setAccessible(true);
        Unsafe unsafe = (Unsafe)unsafeField.get(null);
        while(true){
            unsafe.allocateMemory(_1MB);
        }

    }
}
  • JVM 参数
-Xmx20m -XX:MaxDirectMemorySize=10m
  • 抛出 OOM 异常
"C:\Program Files\Java\jdk1.8.0_144\bin\java" "-javaagent:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\lib\idea_rt.jar=1362:C:\Program Files\JetBrains\IntelliJ IDEA 2017.3.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.8.0_144\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\access-bridge-64.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\cldrdata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jaccess.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\jfxrt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\nashorn.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunec.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunmscapi.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\sunpkcs11.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\ext\zipfs.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jce.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfr.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jfxswt.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\resources.jar;C:\Program Files\Java\jdk1.8.0_144\jre\lib\rt.jar;C:\Users\Heygo\Desktop\JVMDemo\out\production\chapter11" com.atguigu.java.MaxDirectMemorySizeTest
Exception in thread "main" java.lang.OutOfMemoryError
	at sun.misc.Unsafe.allocateMemory(Native Method)
	at com.atguigu.java.MaxDirectMemorySizeTest.main(MaxDirectMemorySizeTest.java:20)

Process finished with exit code 1

JDK8 中元空间直接使用本地内存

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